AT&T Inc. plans to begin its first trials of fifth-generation cellular technology, or 5G, this year as wireless carriers look for higher speeds and more capacity amid the growth of mobile data usage.

Cellular networks have evolved as the use of mobile connections has changed. Originally built for voice calls, they now are largely engineered to carry Internet data. Even voice calls increasingly are being carried in the same way as Web traffic. AT&T and other U.S. carriers have spent billions upgrading their networks...